The present invention relates to a probe assembly for testing electrical devices such as silicon wafers, and more particularly to a high-frequency probe assembly having a bias tee.
High frequency testing of an electrical device-under-test (DUT) is usually accomplished by electrically connecting measurement equipment to a high frequency probe assembly that selectively probes contact points on the DUT. Existing probe assemblies include, for example, needle probes, and microwave probes.
Some measurement equipment is designed to be used repeatedly over time in conjunction with different types of probe assemblies. The measurement equipment, therefore, includes input and output ports for connectivity to the probe assembly. Because coaxial adapters have only recently been able to efficiently deliver signals above 65 GHz, frequently required for testing of today's high-frequency electrical circuits, wafer testing equipment may include ports that connect to a waveguide channel capable of delivering signals above 65 GHz.
Unlike waveguide channels, probes usually deliver a test signal to the DUT through needles. Coaxial cables may be used to provide a shielded test signal. Accordingly, it is not uncommon for a probe assembly to include a transition by which a test signal provided by the measurement equipment through a waveguide channel may transition to a coaxial line for use in testing a DUT.
A waveguide to coaxial transmission line transition typically comprises a waveguide channel into which the tip portion of a transmission line, such as the center conductor of a coaxial cable, may be inserted at a right angle to one of the interior surfaces of the waveguide channel. In a typical implementation a backshort having a reflective face is also included within the waveguide channel. The backshort is usually made of brass or some other reflective material, and is oriented perpendicular to the waveguide channel so as to reflect any alternating signal present within the waveguide channel towards the transmission line. The backshort is preferably located close to the transmission line. If properly positioned, the backshort will reflect the alternating signal within the waveguide into a standing wave pattern so that the alternating signal transitions to the transmission line with minimal signal degradation.
The position of the backshort relative to the center conductor of the coaxial cable should be adjusted to optimized performance in the primary band of the alternating signals present within the waveguide channel. Tuning of the transition is often difficult. At high frequencies, very small deviations from an optimal backshort position may lead to significant signal degradation.
Currently accepted practice is to tune the bias tee by adjusting the transition of the backshort by hand. Traditionally, a backshort that is constructed with a necked-down portion having low tensile strength that can be used as a handle. Conductive epoxy is applied around the perimeter of the backshort, which is then inserted into the waveguide channel. Adjustment of the backshort position within the waveguide channel is accomplished manually. Once the desired location of the backshort is obtained, the epoxy is cured by placing the bias tee in a heater. The handle is broken off and removed from the backshort.
A bias tee is a commonly used element to add a bias offset to the alternating signal within a transmission line, when desired. The bias offset is typically added to the alternating signal by wiring a DC source to the center conductor of a coaxial cable. The DC source may be a voltage source or a current source, as appropriate. Usually the DC signal passes through an inductor so that any alternating signal induced in the coaxial cable is generally isolated from the DC source. The bias tee may be incorporated together with the transition to provide a DC bias offset to the high frequency signal in the coaxial transmission line.
The bias tee and transition assembly may be interconnected with a probe, thereby creating a probe assembly, for testing devices. Existing probe assemblies integrate the bias tee with the probe; that is to say that the bias tee is permanently affixed to the probe. Unlike measurement equipment, however, a particular probe and waveguide assembly is not designed to be used repeatedly on successive types of DUTs, especially with different frequency ranges. Rather, a probe and waveguide combination is specially designed to test multiple copies of a single type of a DUT within a particular frequency range. Also, probes are contacting elements and eventually wear out after a number of uses. Because existing probe assemblies integrate the bias tee within the probe assembly, and because they are typically used for a single type of measurement for a particular DUT, the entire assembly is also discarded with the worn or outdated probe. Because of the aforementioned requisite manual tuning and precise positioning of the backshort, repetitive construction and tuning of bias tees is time consuming.
What is desired, therefore, is an assembly that includes a bias tee and transition that may be reused.